1. Field of the Invention
The present invention relates to an image pickup apparatus for use in an image reading apparatus such as a copying machine, a scanner, etc.
2. Description of the Related Art
Conventionally, a solid-state image pickup apparatus for use in a copying machine etc. includes a plurality of pixels, a scanning circuit, a transfer unit, and an output circuit. The plurality of pixels convert by a photoelectric conversion unit optical signals to electric signals. The scanning circuit sequentially and selectively scans the plurality of pixels in a time series. The transfer unit sequentially transfers the electric signal for each pixel according to a timing signal from the scanning circuit. The output circuit outputs the electric signal transferred from the transfer unit to an output terminal.
Specifically, for example, in a CMOS type sensor, a pixel is configured by a photoelectric conversion unit for converting an optical signal into a charge signal, a charge to voltage conversion unit, and a signal storage unit. A transfer unit is a common output line, and is connected to an output amplifier as an output circuit. With the configuration, a voltage signal stored in the signal storage unit for each pixel is sequentially transferred to the output amplifier through the common output line according to a timing signal from the scanning circuit.
The above-mentioned solid-state image pickup apparatus is described in Japanese Patent Application Laid-Open No. H9-331420. The solid-state image pickup apparatus disclosed by the above-mentioned publication has an effective pixel region in which image information is photoelectrically converted and a region of an optical black pixel (hereinafter referred to as an “OB pixel” in which a photoelectric conversion unit is shielded to output a black reference level. The configuration and the operations of the conventional solid-state image pickup apparatus are described below by referring to the attached drawings.
FIG. 8 shows the configuration of the conventional solid-state image pickup apparatus. The apparatus includes an effective pixel region 1, an OB pixel region 2, a scanning circuit 3, a clock signal 4 for driving the scanning circuit, and a pixel selection signal 5 output from the scanning circuit in synchronization with the clock signal. A pixel selection circuit 6 connects each pixel of the effective pixel region 1 or the OB pixel region 2 to a transfer unit according to the pixel selection signal. The apparatus also includes a transfer unit 7 and an output circuit 8. FIG. 8 shows, for example, an eight-pixel line sensor configured by four OB pixels and four effective pixels.
The operation is described below by referring to the timing chart shown in FIG. 9. The pixel selection signal 5 in synchronization with the first clock signal 4 is output from the scanning circuit 3, and each pixel in the OB pixel region 2 and the effective pixel region 1 is sequentially selected pixel by pixel in a time series as shown in FIG. 9. Thus, the selected pixel and the transfer unit 7 are connected by the pixel selection circuit 6, an electric signal stored in the pixel is transferred to the output circuit 8, and externally output from the circuit. In the example shown in FIG. 9, the output level is held over a cycle of the first clock signal 4, but a signal output level and a reset level can be combined and output in one cycle in a certain solid-state image pickup apparatus.
The output electric signal of the OB pixel region 2 is fetched by a clamping unit and an averaging unit in a signal processing unit at the subsequent stage, and is used as a black reference level. An electric signal of the effective pixel region 1 is input to the signal processing unit at the subsequent stage, and processed as image information.
As described above, the OB pixel output requires a certain stability time because of the operation through the clamping unit etc. for fetching as a black reference level at the subsequent stage. As described above by referring to FIG. 9, when the conventional solid-state image pickup apparatus is used, the effective pixels and the OB pixels are scanned at the same frequency. Therefore, it is necessary to provide the number of OB pixels corresponding to the required stability time, and secure the OB pixel output time, thereby increasing the entire area of chips.
In a certain copying machine etc., a different clock frequency can be used. Therefore, although the same OB pixel output time is required, a larger number of OB pixels are required in a system of a high-speed operation, and a smaller number of OB pixels are required in a system operating at a lower speed. Additionally, depending on the signal processing unit connected at the subsequent stage, a different OB pixel output time can be required. Thus, since a different OB pixel output time can be required depending on the system, a solid-state image pickup apparatus having an OB pixel output time for each system, that is, each number of OB pixels, has to be developed.